This invention relates to encoders for converting translational or rotational mechanical movement into electronic position signals. More particularly, this invention relates to analog-to-digital encoders for converting the physical position of a liquid level measuring instrument into digitally encoded electronic position signals.
U.S. patent application Ser. No. 595,185 filed July 11, 1975 for "ANALOG-TO-DIGITAL ENCODER UNIT EMPLOYING A PRE-ENCODED FILM", to be issued as U.S. Pat. No. 3,975,603 on Aug. 17, 1976, the disclosure of which is hereby incorporated by reference, discloses and claims an optical encoder unit for generating digital electrical signals defining the absolute level of a liquid in a tank from the translational position of a perforated tape in a conventional liquid level gauge, which is an improvement over commercially available tank gauge systems. The invention of the referenced patent employs a pre-encoded film strip having a plurality of laterally spaced longitudinally extending tracks with transparent and opaque regions encoded in a predetermined digital code format, which film strip is received on a pair of freely rotatable spools and is guided past an optical detection station by a pair of spaced stationary guide members. A rotatable sprocket having an input shaft adapted to be driven by a shaft coupled to a sprocket in a conventional perforated tape liquid level gauge drives the film strip past the detection station in response to movement of the tape in either direction. A spring-biased pivotally mounted tension arm having a guide in surface contact with the film strip maintains tension therein as the film strip is moved from reel to reel. The spools are mechanically interlinked with a drive belt to provide film take up in either direction.
The optical detection station includes a light source assembly for generating a plurality of laterally spaced light beams and a detector assembly having a corresponding plurality of photosensitive devices each associated to a different one of the light beams and shielded from the remaining beams. The film strip is positioned so that each track occupies the light path between a different one of the beams and photosensitive devices in order to define a plurality of information channels. The electrical position signals output from the photosensitive devices are coupled to a local or remote readout unit provided with electronic circuitry for converting the signal to operator readable digital display values.
While the above-described referenced invention affords several advantages over conventional tank gauge systems, such as non-volatile storage of the liquid level information, local or remote readout, easy installation and removal, and absolute encoding of the liquid level information over a wide range, the specific embodiment disclosed in the above-referenced application is designed for mounting in a particular attitude which is not always convenient to employ. Further, while relatively compact in size, this embodiment is not suitable for use in extremely limited space.